Most infectious agents have their first contact with the host at a mucosal surface; therefore, mucosal protective immune mechanisms are of primary importance in preventing these agents from colonizing or penetrating the mucosal surface. Numerous studies have demonstrated that a protective mucosal immune response can best be initiated by introduction of the antigen at the mucosal surface, and parenteral immunization is not an effective method to induce mucosal immunity. Antigen taken up by the gut-associated lymphoid tissue (GALT), primarily by the Peyer's patches in mice, stimulates T helper cell (TH) to assist in IgA B cell responses or stimulates T suppressor cells (TS) to mediate the unresponsiveness of oral tolerance. Particulate antigen appears to shift the response towards the (TH) whereas soluble antigens favor a response by the (TS). Although studies have demonstrated that oral immunization does induce an intestinal mucosal immune response, large doses of antigen are usually required to achieve sufficient local concentrations in the Peyer's patches. Unprotected protein antigens may be degraded or may complex with secretory IgA in the intestinal lumen.
One possible approach to overcoming these problems is to homogeneously disperse the antigen of interest within the polymeric matrix of appropriately sized biodegradable, biocompatible microspheres that are specifically taken up by GALT. Eldridge et. al. have used a murine model to show that orally-administered 1-10 micrometer microspheres consisting of polymerized lactide and glycolide, (the same materials used in resorbable sutures), were readily taken up into Peyer's patches, and the 1-5 micrometer size were rapidly phagocytized by macrophages. Microspheres that were 5-10 micrometers (microns) remained in the Peyer's patch for up to 35 days, whereas those less than 5 micrometer disseminated to the mesenteric lymph node (MLN) and spleen within migrating MAC-1+ cells. Moreover, the levels of specific serum and secretory antibody to staphylococcal enterotoxin B toxoid and inactivated influenza A virus were enhanced and remained elevated longer in animals which were immunized orally with microencapsulated antigen as compared to animals which received equal doses of non-encapsulated antigen. These data indicate that microencapsulation of an antigen given orally may enhance the mucosal immune response against enteric pathogens. AF/R1 pili mediate the species-specific binding of E. coli RDEC-1 with mucosal glycoproteins in the small intestine of rabbits and are therefore an important virulence factor. Although AF/R1 pili are not essential for E. coli RDEC-1 to produce enteropathogenic disease, expression of AF/R1 promotes a more severe disease. Anti-AF/R1 antibodies have been shown to inhibit the attachment of RDEC-1 to the intestinal mucosa and prevent RDEC-1 disease in rabbits. The amino acid sequence of the AF/R1 pilin subunit has recently been determined, but specific antigenic determinants within AF/R1 have not been identified.
Recent advances in the understanding of B cell and T cell epitopes have improved the ability to select probably linear epitopes from the amino acid sequence using theoretical criteria. B cell epitopes are often composed of a string of hydrophilic amino acids with a high flexibility index and a high probability of turns within the peptide structure. Prediction of T cell epitopes are based on the Rothbard method which identifies common sequence patterns that are common to known T cell epitopes or the method of Berzofsky and others which uses a correlation between algorithms predicting amphipathic helices and T cell epitopes.
In the current study we have used these theoretical criteria to predict probable T or B cell epitopes from the amino acid sequence of AF/R1. Four different 16 amino acid peptides that include the predicted epitopes have been synthesized: AF/R1 40-55 as a B cell epitope, 79-94 as a T cell epitope, 108-123 as a T and B cell epitope, and AF/R1 40-47/79-86 as a hybrid of the first eight amino acids from the predicted B cell epitope and the T cell epitope. We have used these peptides as well as the native protein to stimulate the in vitro proliferation of lymphocytes taken from the Peyer's patch, MLN, and spleen of rabbits which have received intraduodenal priming with microencapsulated or non-encapsulated AF/R1. Our results demonstrate the microencapsulation of AF/R1 potentiates the cellular immune response at the level of the Peyer's patch, thus enhancing in vitro lymphocyte proliferation to both the native protein and its linear peptide antigens. CFA/I pili, rigid thread-like structures which are composed of repeating pilin subunits of 147 amino acid found on serogroups 015, 025, 078, and 0128 of enterotoxigenic E. coli (ETEC) [1-4, 18]. CFA/I promotes mannose resistant attachment to human brush borders [5]; therefore, a vaccine that established immunity against this protein may prevent the attachment to host tissues and subsequent disease. In addition, because the CFA/I subunit shares N-terminal amino acid sequence homology with CS1, CFA/II (CS2) and CFA/IV (CS4) [4], a subunit vaccine which contained epitopes from this area of the molecule may protect against infection with various ETEC.
Until recently, experiments to identify these epitopes were time consuming and costly; however, technology is now available which allows one to simultaneously identify all the T cell and B cell epitopes in the protein of interest. Multiple Peptide synthesis (Pepscan) is a technique for the simultaneous synthesis of hundreds of peptides on polyethylene rods [6]. We have used this method to synthesize all the 140 possible overlapping actapeptides of the CFA/I protein. The peptides, still on the rods, can be used directly in ELISA assays to map B call epitopes [6, 12-14]. We have also synthesized all the 138 possible overlapping decapeptides of the CFA/I protein. For analysis of T cell epitopes, these peptides can be cleaved from the rods and used in proliferation assays [15]. Thus this technology allows efficient mapping and localization of both B cell and T cell epitopes to a resolution of a single amino acid [16]. These studies were designed to identify antigenic epitopes of ETEC which may be employed in the construction of an effective subunit vaccine.
CFA/I pili consist of repeating pilin protein subunits found on several serogroups of enterotoxigenic E. coli (ETEC) which promote attachment to human intestinal mucosa. We wished to identify areas within the CFA/I molecule that contain immunodominant T cell epitopes that are capable of stimulating the cell-mediated portion of the immune response in primates as well as immunodominant B cell epitopes. To do this, we (a) resolved the discrepancy in the literature on the complete amino acid sequence of CFA/I, (b) immunized three Rhesus monkeys with multiple i.m. injections of purified CFA/I subunit in Freund's adjuvant, (c) synthesized 138 overlapping decapeptides which represented the entire CFA/I protein using the Pepscan technique (Cambridge Research Biochemicals), (d) tested each of the peptides for their ability to stimulate the spleen cells from the immunized monkeys in a proliferative assay (e) synthesized 140 overlapping octapeptides which represented the entire CFA/I protein, and (f) tested serum from each monkey for its ability to recognize the octapeptides in a modified ELISA assay. A total of 39 different CFA/I decapeptides supported a significant proliferative response with the majority of the responses occurring within distinct regions of the protein (peptides beginning with residues 8-40, 70-80, and 127-137). Nineteen of the responsive peptides contained a serine residue at positions 2, 3, or 4 in the peptide, and a nine contained a serine specifically at position 3. Most were predicted to be configured as an alpha holix and have a high amphipathic index. Eight B cell epitopes were identified at positions 3-11, 11-21, 22-29, 32-40, 38-45, 66-74, 93-101, and 124-136. The epitope at position 11-21 was strongly recognized by all three individual monkeys, while the epitopes at 93-101, 124-136, 66-74, and 22-29 were recognized by two of the three monkeys.